An electronic component such as a semiconductor device is usually mounted on a circuit substrate. In general, the circuit substrate comprises an insulating base material and a conductive pattern. Examples of the insulating base material that can be used in the circuit substrate not only include: resin materials such as Bakelite; impregnation materials such as a paper phenol material made by hardening paper with a phenolic resin, and a glass epoxy material made by hardening glass fiber with an epoxy resin; but also include ceramics and glass.
The amount of heat generated by an electronic component, especially a semiconductor device, to be mounted onto a circuit substrate has increased as the performance of the semiconductor device has been enhanced. It is therefore desired to form a circuit substrate from a material that has higher thermal conductivity and a heat radiating capability. In many cases, ceramic substrates, a representative example of which is an alumina sintered body, are used as the circuit substrate material having high thermal conductivity. These days, there are also considerations of using an aluminum nitride sintered body, which has higher thermal conductivity than the alumina sintered body.
In order to produce a circuit substrate by using a sintered nitride ceramic substrate, a typical example of which is this aluminum nitride sintered body, it is necessary to form metal wiring on a surface of the nitride ceramic sintered body. There are severable methods for forming metal wiring, such as a thick-film method in which to apply a metal paste and thereafter perform firing; and a thin-film method in which to form a metal thin film by vapor deposition. Especially in the uses where the heat radiating capability is required, a large amount of electric current is often needed. As the thickness of the film formed by the thin-film method causes limitations on an electric current permitted to flow, the thick-film method is favorably adopted.
Further in the thick-film method, printing methods such as a screen printing method and an ink jet printing method can be adopted as the method of applying the metal paste. Therefore, it is possible to define a shape of a wiring pattern concurrently with forming a layer of a metal material. That is, it is unnecessary to carry out etching treatment and the like after forming the layer of a metal material in order to define the shape of the wiring pattern. As such, costs can be easily reduced, and in view of the cost reductions as well, the thick-film method is favorably adopted.
The following are known as an industrial method for forming metal wiring on a ceramic substrate by the thick-film method: a co-firing method and a post-firing method in which a paste containing a high-melting-point metal powder is used. The co-firing method is a method in which to apply a high-melting-point metal paste onto a ceramic green sheet and fire them to thereby concurrently carry out sintering of the ceramic and firing of the high-melting-point metal. The co-firing method has characteristics that although it enables a solidly adhered metal layer to be formed, it causes difficulty in forming a metal pattern with high dimensional precision due to the shrinkage of the ceramic that results from the sintering thereof. The post-firing method is a method in which to apply a high-melting-point metal paste onto a pre-sintered ceramic substrate and thereafter fire it. This method basically does not entail such a problem of dimensional precision as seen in the above co-firing method. However, especially in the nitride ceramics, a typical example of which is aluminum nitride, it has been difficult to attain high bonding strength (adhesion strength) of the metal layer by the post-firing method. Accordingly, there has recently been suggested a technique (Patent Document 1) of improving the adhesion strength by using a paste containing a Ag—Cu alloy as a main component and titanium hydride as a secondary component.